Others above have mentioned the Wise Pilgrim and Buen Camino apps, which seem to be the most commonly recommended. Both come with map functions. For a while, Camino Ninja was giving them some good competition but sadly the person behind the app passed away and it seems to be discontinued. I use all of those, and also used Camino Pilgrim. They are inexpensive enough and I have enough room on my phone so that it isn't an issue. Then I can check them against each other.
Some like to use another mapping app and download the Camino routes to import into those apps. There are several sources for the Camino Routes to download including the Centro Nacional de Información Geográfica (CNIG):
http://centrodedescargas.cnig.es/CentroDescargas/loadCamSan.do# and a Dutch site run by camino enthusiasts. I like to import those routes into a Google map and add markers for recommended albergues, bars, restaurants, etc.
I also like to also carry Kindle copies of some guidebooks on my phone. The
Brierley guides do not seem to be available in this format, but the abbreviated map books with maps and albergue listings are. The Village to Village guides and the Moon guide by Beebe Bahrani are available as e-books. The maps in these e-books aren't interactive like the apps, though. More importantly,
The Pilgrimage Road to Santiago: The Complete Cultural Handbook by Gitlitz and Davidson is available as an e-book. It's information isn't in the other guides or apps.
Finally, there are some websites that can be useful for a self-guided trip. Gronze.com is probably the most frequently recommended of these. It is entirely in Spanish but your browser may translate it for you (my Chrome browser does).